Trinickel aluminide-base heat-resistant alloy

ABSTRACT

An Ni 3 Al-base heat-resistant alloy containing, in % by weight, 6.0 to 9.0% of Al, 2.0 to 15.0% of Cr and 0.5 to 3.0% of Zr, the balance being Ni and inevitable impurities, the alloy having a metal structure comprising Ni 3 Al as the main phase thereof. When required, the alloy may further contain over 0% to not more than 5.0% of W, over 0% to not more than 3.0% of Mo, over 0% to not more than 3.0% of Nb, over 0% to not more than 0.003% of B, over 0% to not more than 0.3% of C and 0.003 to 0.03% of N, wherein the combined amount of W, Mo and Nb is up to 5.0% if at least two of these elements are present.

FIELD OF THE INVENTION

[0001] The present invention relates to Ni₃Al-base heat-resistant alloys having desired high-temperature strength (tensile strength and proof stress) and excellent in creep rupture strength and weldability.

BACKGROUND ART

[0002] Trinickel aluminide (Ni₃Al) is an intermetallic compound (about 1390° C. in melting point) having a face-centered cubic crystal structure and possesses the peculiar property that the yield strength thereof, which is indicative of the strength of the material, increases with a rise in temperature.

[0003] Because of this property, Ni₃Al-base heat-resistant alloys, i.e. alloys containing precipitating Ni₃Al dispersed therein are capable of retaining a high stress level in a temperature range of up to high temperatures. Such Ni₃Al-base heat-resistant alloys heretofore proposed include the following.

[0004] Japanese pre-examination publication 62-93334 discloses an alloy which comprises, in atomic %, 0.2 to 1.5% of a Group IVb element (Zr or Hf), 17 to 20% of Al, 4.5 to 8% of Cr, 0.05 to 0.2% of B, 9 to 16% of Fe, 0.001 to 0.004% of a rare-earth element (such as Ce) and the balance Ni and which is improved in strength at high temperatures, ductility and hot workability.

[0005] Japanese post-examination publication 63-66374 discloses an Ni-Al alloy consisting mainly of Ni₃Al, containing, in wt. %, 0.01 to 2.0% of Mo, 0.05 to 3.0% of B, 0.5 to 4.0% of Zr, etc. and improved in ductility at room temperature and strength.

[0006] Japanese pre-examination publication 63-266036 discloses an Ni-base alloy having a metal structure wherein Ni₃Al is its main phase, containing, in atomic %, 75.4 to 79% of Ni, 7 to 12% of Al, up to 0.5% of B, up to 0.9% of C, 0.5 to 4% of Hf, 4.5 to 11% of Fe, up to 3% of Mo, W, Nb or Zr and improved in ductility at room temperature, strength, etc.

[0007] Japanese national phase publication 4-501440 discloses an alloy containing, in atomic %, 15 to 18.5% of Al, 6 to 10% of Cr, 0.05 to 0.35% of Zr, 0.08 to 0.30% of B and the balance Ni, and improved in ductility at high temperatures, workability and strength.

[0008] Japanese Patent No. 2599263 discloses an Ni-Al alloy comprising Ni₃Al as its main phase, containing, in wt. %, less than 1% of a Group IVb element (Hf or Zr), 14.5 to 17.5% of Fe, up to 0.01% of a rare-earth element (such as Ce, Y or La), 0.01 to 0.05% of B and up to 4% of Mo and improved in workability.

[0009] However, since Ni₃Al exhibits almost no elongation, the conventional Ni₃Al-base heat-resistant alloys have the problem of becoming markedly impaired in creep rupture strength in a high-temperature range exceeding 1050° C., and are therefore limited in application to high-temperature use from the viewpoint of strength.

[0010] Further because the intermetallic compound Ni₃Al is highly susceptible to weld cracking, such alloys are not usable for structural bodies which require welding for assembling. Thus, the conventional alloys are limited in use as heat-resistant alloys.

[0011] An object of the present invention, which has been accomplished in view of the foregoing problems, is to provide an Ni₃Al-base heat-resistant alloy having a high creep rupture strength in a high-temperature range in excess of 1050° C. and excellent weldability.

SUMMARY OF THE INVENTION

[0012] The present invention provides a heat-resistant alloy containing, in % by weight, 6.0 to 9.0% of Al, 2.0 to 15.0% of Cr and 0.5 to 3.0% of Zr, the balance being Ni and inevitable impurities, the alloy having a metal structure comprising Ni₃Al as the main phase thereof.

[0013] The expression “Ni₃Al as a main phase” as used herein means that the proportion by volume of Ni₃Al in the metal structure the main phase of which is an Ni solid solution is larger than 50%. To assure the desired high-temperature strength (proof stress and tensile strength), the proportion of Ni₃Al by volume is preferably at least 70%.

[0014] When required, over 0% to not more than 5.0% of W, over 0% to not more than 3.0% of Mo, over 0% to not more than 3.0% of Nb, over 0% to not more than 0.003% of B, over 0% to not more than 0.3% of C and 0.003 to 0.03% of N can be incorporated into the heat-resistant alloy of the present invention. When at least two of W, Mo and Nb are present, the combined amount of these elements is up to 5.0%.

[0015] Most preferably, the heat-resistant alloy of the present invention contains Al, Cr, Zr, W, B, C and/or N as effective elements in the respective ranges described, the balance being substantially Ni.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a photograph showing sample No. 12 as subjected to a weldability test involving a dye check, and

[0017]FIG. 2 is a photograph showing sample No. 102 as subjected to a weldability test involving a dye check.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The components of the Ni₃Al-base heat-resistant alloy of the invention are limited for the reasons given below.

[0019] In the following description, the percentages are all by weight.

[0020] Al: 6.0-9.0%

[0021] Al is a basic element for forming the intermetallic compound of Ni₃Al along with Ni. If the content of Al is less than 6.0%, an insufficient quantity of Ni₃Al phase will result, failing to afford the desired high-temperature strength. If the content is over 9.0%, on the other hand, the alloy is lower in creep rupture strength. Accordingly, the Al content should be 6.0 to 9.0%.

[0022] Cr: 2.0-15.0%

[0023] Cr contributes to an improvement in tensile strength at high temperatures. This effect is available when the Cr content is at least 2.0%. However, the presence of more than 15.0% of Cr entails an excessively increased hardness to result in impaired ductility at room temperature. For this reason, the Cr content should be 2.0 to 15.0%, preferably 4.0 to 8.0%.

[0024] Zr: 0.5-3.0%

[0025] Zr is effective for giving remarkably improved weldability. Since the Ni₃Al-base heat-resistant alloy comprising Ni₃Al as its main phase is highly susceptible to weld cracking, the alloy is liable to develop weld cracks when welded. However, the presence of Zr forms Ni₅Zr in the base phase, affording improved resistance to cracking at the grain boundary. Zr further contributes to improvements in strength (tensile strength and proof stress) and creep rupture strength at high temperatures in excess of 1050° C. Accordingly, at least 0.5%, preferably at least 1.0%, of Zr should be present. However, if the content is over 3.0%, the effect levels off, so that this value is the upper limit.

[0026] The heat-resistant alloy of the present invention comprises Ni and inevitable impurities providing the balance. With the Ni₃Al-base heat-resistant alloy of the invention, the impurities include, for example, Fe in addition to S, P, etc. Preferably, the alloy of the invention should be minimized in Fe content since this element produces an impaired strength at high temperatures. If the content exceeds 3.0%, an unnegligible influence will result, so that the content should be limited to not greater than 3.0%, preferably to not greater than 1.0%.

[0027] When required, the following elements can be incorporated into the Ni₃Al-base heat-resistant alloy of the present invention.

[0028] W: over 0% to not more than 5.0%

[0029] W is an element effective for giving a higher creep rupture strength at high temperatures. However, the presence of an excess of this element not only leads to lower tensile ductility at high temperatures but also conversely entails an impaired creep rupture strength while producing the influence of lower weldability. The W content should therefore be up to 5.0% if highest and is preferably 0.5 to 5.0%, more preferably 1.0 to 4.0%.

[0030] Mo, Nb: over 0% to not more than 3.0%

[0031] Like W, Mo and Nb are elements effective for giving a higher creep rupture strength at high temperatures. Mo and Nb can therefore be made present in place of or along with W. Like W, however, an excessive Mo or Nb content not only leads to lower tensile ductility at high temperatures but also conversely entails an impaired creep rupture strength while producing the influence of lower weldability. For this reason, the Mo and Nb contents should each be up to 3.0%.

[0032] W, Mo and Nb are similar in effect, so that when at least two of these elements are used, the combined amount of these elements should be limited to not greater than 5.0%.

[0033] C: over 0% to not more than 0.3%

[0034] When C is present conjointly with Cr, Cr carbide precipitates at the grain boundary to fortify the boundary, giving improved ductility at high temperatures. This effect is available when a trace quantity of C is present. If the C content exceeds 0.3%, impaired ductility will result in the range of ordinary temperatures along with lower weldability. Accordingly, the C content should be limited to not greater than 0.3% and is preferably 0.1 to 0.2%.

[0035] N: 0.003-0.03%

[0036] Like C, N acts to give improved ductility at high temperatures. This effect appears when at least 0.003% of N is present. Although an increase in content affords an enhanced effect, presence of more than 0.03% of N entails impaired economy in producing the alloy by melting. The N content should therefore be 0.003 to 0.03% and is preferably 0.004 to 0.02%,

[0037] B: over 0% to not more than 0.003%

[0038] B segregates at the grain boundary, giving enhanced ductility and contributing to an improvement in creep rupture strength at high temperatures. However, presence of more than 0.003% of B results in enhanced susceptibility to weld cracking and seriously impaired weldability. For this reason, the B content should be limited to not greater than 0.003% and is preferably 0.001 to 0.002%.

[0039] The metal structure of the Ni₃Al-base heat-resistant alloy of the invention contains an Ni solid solution as its base phase and has the main phase of Ni₃Al and a small amount of Ni₅Zr precipitate phase as mixed therewith.

EXAMPLE

[0040] Samples were prepared by the following procedure.

[0041] An alloy was produced by high-frequency melting in an argon atmosphere using an alumina crucible (145 mm in inside diameter and 256 mm in height). First, Ni was melted by heating, Al was added to the Ni as melted, and the mixture was heated to a higher temperature. Specified metal elements were then added to the mixture, followed by temperature adjustment, and the resulting melt was poured into a ladle. The melt was 16 kg in weight.

[0042] Next, a tubular sample (137 mm in outside diameter, 19 mm in wall thickness and 270 mm in length) was prepared by centrifugal casting in the atmosphere using a metal mold.

[0043] The macroscopic structure of the tubular sample as cast centrifugally was found to contain columnar crystals on the outer side and granular crystals locally present on the inner side owing to directional solidification.

[0044] Specified test pieces were made from each of samples thus prepared, and subjected to a high-temperature characteristics test, a high-temperature creep rupture test and a weldability test. The high-temperature characteristics test was conducted to check for tensile strength, 0.2% proof stress, elongation or reduction of area at 1100° C. The high-temperature creep rupture test was conducted to determine the creep rupture time at 1100° C. under a load of 30 MPa. For the weldability test, the test piece was TIG-welded on its outer periphery axially thereof by the beads-on-plate method (welding current 120 A) and thereafter checked with the unaided eye for cracks as dyed.

[0045] Table 1 shows the chemical compositions of alloys of the samples and the results of the tests. TABLE 1 Alloy Chemical Composition High temperature characteristics Creep (Balance being Ni and inevitable impurities) Tensile 0.2% Proof Elon- Reduction ruture Sample (wt. %) strength stress gation of area time No. Al Cr Zr W Mo Nb C N B Fe (MPa) (MPa) (%) (%) (Hrs.) Weldability 1 7.5 5.0 1.8 — — — — — — 0.09 158  95 25.9 30.6 90.2 ◯ 2 7.5 2.5 1.0 2.5 — — — — — 0.90 233 139 5.1 5.1 70.2 ◯ 3 7.5 5.0 1.5 — 1.5 — — — — 0.10 194 116 21.2 44.7 69.3 ◯ 4 7.5 5.0 1.8 — — 1.0 — — — 0.10 245 127 3.2 3.4 66.0 ◯ 5 8.0 7.0 1.8 — — — 0.25 — — 0.10 191 110 9.4 11.7 71.4 ◯ 6 8.0 6.5 1.8 — — — — 0.0045 — 0.10 201 114 5.1 6.3 85.2 ◯ 7 7.5 5.0 1.2 — — — — — 0.0015 — 149  99 24.0 31.3 113.4 ◯ 8 7.5 5.0 1.5 2.5 — — — 0.0057 — 0.90 225 129 6.1 6.1 110.2 ◯ 9 7.5 5.0 1.2 2.5 — — 0.05 0.0052 — 0.90 199 112 3.2 9.9 108.6 ◯ 10 7.5 5.0 1.5 — 2.0 — — — 0.0020 0.10 193 113 16.6 29.0 77.0 ◯ 11 7.5 5.0 1.5 — — — 0.15 — 0.0015 0.10 172 104 17.4 23.3 62.3 ◯ 12 7.5 5.0 1.8 2.5 — — 0.15 — 0.0015 0.90 184 108 11.2 10.5 147.9 ◯ 13 7.5 10.0 1.8 2.5 — — 0.15 0.0120 0.0015 0.90 230 158 15.8 24.6 160.5 ◯ 14 7.5 12.5 1.5 — 1.0 1.5 — 0.0140 — 0.10 212 111 3.5 3.9 55.9 ◯ 15 7.5 5.0 1.5 2.0 1.0 1.5 0.08 0.0060 0.0015 0.90 236 131 3.3 3.7 50.5 ◯ 101 7.5 5.0 — 2.5 — — — — — 0.90  83  37 0.5 2.2 0.2 X 102 8.0 5.5 0.2 — — — — — — 0.10  95  62 2.6 2.0 0.3 X 103 7.5 5.0 1.5 2.5 1.5 1.5 — — — 0.90 245 135 1.8 1.6 26.0 Δ 104 5.0 5.0 1.8 — — — — — — 0.10  54  26 11.9 10.1 0.5 ◯ 105 10.0 5.0 1.8 — — — — — — 0.10 169 — 0 0 2.1 ◯ 106 8.0 5.0 1.5 — — — 0.35 — — 0.10 157  90 12.0 13.6 88.7 X 107 8.0 5.0 1.8 — — — — — 0.0045 0.10 207 115 6.0 6.9 93.7 X 108 7.5 5.0 1.2 7.0 — — — — 0.0015 — 247 210 1.8 1.1 24.0 Δ 109 7.5 5.0 1.5 2.5 — — 0.15 — 0.0015 5.50 166 101 3.1 4.4 8.4 ◯

[0046] In Table 1, samples No.1 to No. 15 are examples of the invention, and samples No.101 to No. 109 are comparative examples. Incidentally, Fe is handled as an impurity element in the case of the alloy of the invention. When the ingredients are melted for the preparation of the alloy, Fe is liable to become incorporated into the alloy from the materials, producing an adverse effect on the strength if present in an increased amount. Accordingly, Table 1 shows Fe contents.

[0047] In the weldability column of Table 1, the circular mark indicates “no cracking”, the triangular mark “slight cracking”, and the cross mark “marked cracking”.

[0048] The results of Table 1 reveal that the examples of the invention are superior to the comparative examples when the high-temperature characteristics, high-temperature creep rupture strength and weldability of the samples were evaluated collectively.

[0049] Among the comparative examples, No. 106 and No. 107 are satisfactory in creep rupture strength but low in weldability. Although No. 104 and No. 105 have good weldability, No. 104 is low in high-temperature strength (tensile strength and proof stress) and in creep rupture strength, and No. 105 is poor in high-temperature elongation and creep rupture strength. No value is given as the 0.2% proof stress of No. 105 because the test piece ruptured immediately when tested, hence the value was immeasurable. No. 109 exhibited a low creep rupture strength due to a high Fe content.

[0050] The test piece of example of the invention, No.1 was polished by buffing, and corroded with Marble's reagent. When observed under an electron microscope (magnification: X5000), the metal structure was found to contain 88% by volume of the intermetallic compound, Ni₃Al.

[0051]FIGS. 1 and 2 show photographs of the test pieces of the invention example, No. 12 and comparative example, No. 102, as tested for weldability and dyed for checking. The dyed portions in FIG. 2 indicate cracks occurred.

[0052] The Ni₃-Al-base heat-resistant alloy of the present invention is suitable, for example, as radiant tubes for use in steel material heating furnaces, hearth rolls for use in heating furnaces and cracking tubes for use in pyrolysis furnaces in the field of the petrochemical industry. These products are fabricated usually by preparing the alloy by the melting process and casting the alloy. Such products are usable of course as cast, but can be subjected to a solution heat treatment at a temperature of about 1100 to about 1200° C. when so required. This treatment is expected to make the product more uniform in structure and further improved in properties.

[0053] The alloy of the invention is usable in the form of a powder as a cladding material for forming clad structures. Cladding can be performed, for example, by the plasma powder welding method (PPW method). 

What is claimed is:
 1. A heat-resistant alloy excellent in creep rupture strength at high temperatures and in weldability and consisting of, in % by weight, 6.0 to 9.0% of Al, 2.0 to 15.0% of Cr and 0.5 to 3.0% of Zr, the balance being Ni and inevitable impurities, the alloy having a metal structure comprising Ni₃Al as the main phase thereof.
 2. The heat-resistant alloy according to claim 1 which contains one element selected from the group consisting of over 0% to not more than 5.0% of W, over 0% to not more than 3.0% of Mo and over 0% to not more than 3.0% of Nb.
 3. The heat-resistant alloy according to claim 1 which contains up to 5.0%, in a combined amount, of at least two elements selected from the group consisting of over 0% to not more than 5.0% of W, over 0% to not more than 3.0% of Mo and over 0% to not more than 3.0% of Nb.
 4. The heat-resistant alloy according to claim 1 which contains over 0% to not more than 0.3% of C and/or 0.003 to 0.03% of N.
 5. The heat-resistant alloy according to claim 2 which contains over 0% to not more than 0.3% of C and/or 0.003 to 0.03% of N.
 6. The heat-resistant alloy according to claim 3 which contains over 0% to not more than 0.3% of C and/or 0.003 to 0.03% of N.
 7. The heat-resistant alloy according to claim 1 which contains over 0% to not more than 0.003% of B.
 8. The heat-resistant alloy according to claim 2 which contains over 0% to not more than 0.003% of B.
 9. The heat-resistant alloy according to claim 3 which contains over 0% to not more than 0.003% of B.
 10. The heat-resistant alloy according to claim 4 which contains over 0% to not more than 0.003% of B.
 11. The heat-resistant alloy according to claim 5 which contains over 0% to not more than 0.003% of B.
 12. The heat-resistant alloy according to claim 6 which contains over 0% to not more than 0.003% of B. 